1. Field of the Invention
This invention relates to a fuel tank that is intended to remain in a filled condition for a long period of time. More particularly, a metallic seal that is unitary with the walls of the fuel tank isolates a fluid fuel from elastomeric components that may deteriorate from extended contact with the fuel.
2. Description of Related Art
Spacecraft power plants, such as attitude control engines and thrusters, that are used in extended time missions, for example interplanetary or extraplanetary exploration, frequently remain inactive for an extended time, such as years. At the end of this period of inactivity, a fuel is delivered to the power plant and that component performs its mission.
Contact with typical spacecraft fuels, such as monomethylhydrazine, cause the elastomeric components to corrode, or otherwise deteriorate, over time. Accordingly, the fuel is contained in a fuel tank that isolates the fuel from the elastomeric components during storage. Fuel is then delivered to the power plant at an appropriate time. In one method of delivery, a piston generates a pressure effective to deliver the fuel. Elastomeric components of the piston, such as pressure retaining 0-rings, are also subject to corrosion by contact with the fuel.
One approach to isolate the fuel from elastomeric components is to weld a thin metallic sheet to the inner wall of the fuel tank between the piston and the liquid fuel as an isolation seal. Actuation of the piston ruptures the isolation seal enabling the piston to deliver fuel to the power plant. The elastomeric components are exposed to fuel only during the power plant mission life, typically a few minutes or a few hours. Relatively short term exposure to the fuel reduces corrosion of elastomeric components to acceptable levels.
A welded isolation seal is disclosed in U.S. Pat. No. 5,385,081 to Sneddon that is incorporated by reference in its entirety herein.
It has proven difficult to reliably weld to an inside wall of a fuel tank and pinhole leaks in the weld enable the fuel to contact elastomeric components endangering reliable operation of the power plant. The weld integrity may impact the reliability of the isolation seal making it difficult to predict the force required for the piston to rupture the seal. Welding also increases processing costs and reduces the robustness of the fuel tank. While it is possible to evaluate the integrity of the weld by x-ray analysis, this analysis is time-consuming and expensive.
Accordingly, there remains a need for a fuel tank that isolates fuel from the elastomeric components that does not have the disadvantages of a welded isolation seal.